Method for moulding an article from plastic material

ABSTRACT

An article (11) of thermoplastic material is moulded from a parison (10) in which certain parts (17,16) of the parison are re-formed while other parts (13,12,20) retain their shape. A difference in pressure is applied to the wall of the parison during the re-forming process. Material in the parison is also moved in the direction of those parts of the parison which are to be re-formed, whereby attenuation of the material may be avoided entirely, if so desired, also in those parts which are to be re-formed. The invention enables the parison for a bottle to be re-formed to an extent which is small by comparison with the thickness of the material, for example the so-called &#34;neck support&#34; ring and the so-called &#34;pilfer proof&#34; ring, starting with a tube of oriented material. In a device for the application of the invention the elements (22,21,25) are in contact with the parts (13,12,20) of the parison. Between the elements, which are movable relative to each other, are to be found depressions (27,26) in which the parts (17,16) are re-formed. When the elements (22,21,25) are moved towards each other, the size of the depressions is reduced in the direction of the movement.

FIELD OF THE INVENTION

This invention relates to a method for moulding an article of plasticmaterial from a parison of arbitrary form and where said parison isre-formed into the finished article by means of a difference in pressureapplied to the wall of the parison and where in the re-forming of theparison an initial number of areas of the material are acted upon byelements which move these initial areas of the material in the parison,thereby creating the possibility for these other areas of the materialto be re-formed when the article is moulded. The invention also relatesto a device operating in accordance with the method indicated above formoulding a tube-shaped article.

BACKGROUND

When moulding articles from parisons of plastic material, certainapplications may present problems in achieving the desired re-forming ofthe parison where the extent of the re-forming is small in comparisonwith the wall thickness of the parison. It may be necessary to applyvery great differences in pressure to the wall of the parison,particularly in the case of parisons made from plastic material whichhas already been oriented. Without in any way restricting the framing ofthe problem, an example may be given of the requirement to produceannular protruberances in a tube-shaped parison where after the parisonhas finally been re-formed into the desired article the annularprotruberances constitute a "neck support" ring or a "pilfer proof" ringin a bottle.

In other applications where the extent of the re-forming is great bycomparison with the wall thickness and particularly in the case ofplastic material which has not been oriented, existing methods enablere-forming to take place with the use of relatively small differences inpressure. When the re-forming involves extensive stretching of thematerial, then the wall thickness may occasionally be reduced to such anextent that the strength or the dimensional stability of the mouldedarticle is put at risk. The only solution to this problem with existingmethods has been by the use of base material of adequate thickness.

When moulding a bottle, the requirement exists to produce a neckincorporating a so-called "pilferproof" ring and/or a so-called "necksupport" ring and where in certain applications the "neck support" ringis so shaped that it forms a so-called gripping ring at the same time.

One of the purposes of the gripping ring is to act as a support for thehand so that it is easier to hold the bottle firmly, for instance whenit is being opened. In order to meet this requirement, the gripping ringhas a diameter which is considerably greater than the diameter of theneck at the point of the neck at which the gripping ring is located.Additional cost is associated with the dimensions of the gripping ringbecause it increases the total amount of material required to mould theneck when existing methods are used. This is due to theover-dimensioning of the neck which is necessary in order to provide thegripping ring with adequate strength, for example when pressurizedliquids such as carbonated drinks are being stored. As a rule, thegripping ring also has an undesirably large axial dimension due to themoulding problems indicated above.

One additional requirement for the rings described above is that theyshould exhibit dimensional stability even when heated to relatively hightemperatures, for example so that filled containers may be stored athigh temperatures and so that containers may be cleaned before re-use,etc. It is difficult to achieve this by existing methods withouttreating the material in such a way that its visual appearance changesin an undesirable manner, for example by becoming opaque, whilst thematerial in the remainder of the bottle remains clear and transparent.

One other disadvantage of existing methods is that in the case ofcoloured material those parts of the bottle where there is variation inthe thickness of the material are either darker or lighter than theremainder of the bottle depending on whether there is accumulation orattenuation of the material.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for mouldingan article in plastic material from which the problems referred to abovehave been eliminated. The basis of the invention is the use of a parisonof arbitrary form in which certain areas of the parison are re-formedwhilst other areas of the parison retain their original form to a verygreat extent. When moulding the initial areas of the material the otherareas of the material are moved towards the initial areas, which makesit possible to avoid all attenuation of the wall thickness, ifnecessary, during the final moulding process. The invention will alsopermit an increase in wall thickness to be achieved in those areas whichare re-formed. Even where the material in the parison has been oriented,only relatively small forces will be required for re-forming to takeplace, since the re-forming process occurs without the material beingstretched beyond the point to which it was stretched in conjunction withthe orienting process. This invention makes it possible to re-formoriented material for which, on the assumption that re-forming were tobe at all possible, existing methods would require the use ofunreasonably high differences in pressure. Experiments have shown thatthe material will fracture easily at such high differences in pressure.

When this invention is used in the manufacture of a bottle the startingpoint is a tube-shaped parison in which the areas of the material in theparison which form the neck of the bottle are already oriented and havepreferably been oriented uniaxially. In those applications in which arequirement exists for dimensional stability to be present in thefinished article even at high temperatures, the internal stresses whichwere created during the orienting process may be relieved by heattreatment. By virtue of this invention those areas of the material whichare located adjacent to the material which will become the ring which isabout to be moulded are moved during the moulding of the respective ringtowards that area of the material which makes up the ring after themoulding process is complete. Material is thus brought to the area wherethe ring is moulded, thereby determining the wall thickness of the ring.The ring is moulded without any significant stretching of the material,but by means of a process which resembles folding. This movement of thematerial means that the rings are moulded with the internal pressure ofthe parison at a relatively low level. Where moulding is possible byexisting methods, on the other hand, this requires unreasonably highpressures to be used which can easily produce fractures in the material.

When articles are to be shaped into preforms for bottles, a tubularparison of preferred shape is used, whose central areas have beenoriented uniaxially by stretching the tube in the direction of the axisof the tube. When the parison is re-formed, the central areas aremoulded into two opposing necks in the preforms.

In a preferred embodiment of the invention, the re-forming of theparison occurs at a temperature in excess of the glass-transitiontemperature (TG). In the event of the parison being formed into anarticle which must then undergo further moulding there is an additionalrequirement for the moulding of the article to take place at atemperature in excess of the temperature at which the subsequentmoulding of the article will take place.

This invention is particularly suitable for use with the plasticmaterial polyethylene terephthalate or with similar materials. Of thepolyesters and polyamides, the following materials may be regarded asbeing of interest in the application of this invention:polyhexamethylene adipamide; polycaprolactam; polyhexamethylenesebacamide; polyethylene 2,6- and 1,5-naphthalate; polytetramethylene1,2-dioxybenzoate and the copolymers of ethylene terephthalate, ethyleneisophthalate and other similar polymer plastics.

When using polyethylene terephthalate, the re-forming of the parisontakes place with the material of the parison at a temperature within therange of approximately 70°14 150° C., and preferably within the range ofapproximately 90°14 125° C. This temperature is well in excess of theglass-transition temperature (TG). The material of the parison isoriented by having been stretched in the respective direction of theaxis, preferably about three times. In certain applications, for examplewhen tube-shaped parisons are used as the base material for themanufacture of preforms for bottles, the material is of courseuniaxially oriented.

The recommended values indicated above in respect of temperature andstretching when using polyethylene terephthalate shall be amended whenusing any of the other materials referred to above, depending on thecharacteristics of said material as regards the glass-transitiontemperature and the maximum crystallization temperature, etc.

In a device for moulding a tube-shaped article from a tube-shapedparison, the parison is surrounded by an outer mould in which a numberof retaining elements are so positioned as to secure the parison outsidethe area of the parison which is re-formed when the article is moulded.These retaining elements may be moved relative to each other along theaxis of the mould. Between the retaining elements the outer mould alsohas a number of elements which are in contact with an initial numbers ofareas of the material in the parison. These elements may also be movedrelative to each other along the axis of the mould and may also be movedrelative to the retaining elements. Between the elements which are incontact with the initial areas of the material are depressions in thewall of the mould in which details of the article are moulded from anumber of other areas of the material in the parison, for exampleannular protuberances corresponding to the aforementioned "neck support"rings or "pilfer proof" rings. When the article is being moulded, theelements are in such close contact with the initial areas of thematerial referred to above that, when the elements are moved relative toeach other along the axis of the mould, the areas of the material aremoved in the direction of those areas of the material from which thedetails of the article are moulded. The distance over which eachrespective area of the material is moved corresponds on the whole withthe protuberance in the wall of the parison which is produced as therespective details are moulded.

In many applications the strength of the parison is sufficient for themoulding described above to take place without the use of mechanicalrestraints inside the tube. The excess pressure present inside the tubecontributes to the stability of the tube. A certain reduction in theinternal volume of the tube also occurs during moulding causing theinternal pressure to increase, which in turn both increases thedimensional stability of the parison and contributes to the achievementof the internal pressure required for the re-forming of the parison.

Certain applications have used an internal mandrel which operates inconjunction with the outer mould. The mandrel is fitted withspring-loaded elements for reducing the length of the mandrel in thedirection of the axis. The reduction in the length of the mandrelcorresponds with the reduction in the length of the outer mould when thearticle is moulded.

Between the elements in contact with the initial areas of the materialare arranged spring-loaded elements in which the spring tension variesalong the direction of the axis of the mould. In this way, thespring-loaded elements will be compressed in turn in accordance with apre-determined pattern so that the wall of the parison will be moved inthe order in which the annular protuberances are moulded. The heating ofthe material in the parison is effected either before the parison isplaced in the mould and/or by means of heating devices preferablylocated inside the mandrel. An insulating layer is to be found on themandrel, said insulating layer delineating the one or more areas of themandrel at which heating takes place. The mould is provided with coolingducts. The cooling ducts and the insulating layer separate those areasof the parison where heating takes place from other areas of theparison.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in relation to the annexed drawing containinga number of Figures, in which:

FIGS. 1a-c illustrate the basic sequence of events when moulding anarticle;

FIGS. 2a-c diagrammatically outline the function of a device inaccordance with this invention;

FIGS. 3 & 4 show, in section, a device operating in accordance with thefunction outlined in FIG. 2 with the device in various operatingpositions and where the Figures correspond with Section B--B in FIG. 6;

FIGS. 5a-c show a detailed section through the mandrel shown in FIGS. 3& 4;

FIG. 6 is a sectional view taken along Section C--C in FIG. 3; and

FIG. 7 corresponds to Section A--A in FIGS. 3 & 4, also with thesupporting framework for the device in accordance with FIGS. 3 & 4.

DETAILED DESCRIPTION

FIGS. 1a-c illustrate the principle according to which the re-forming ofa parison 10 into an article 11 takes place. FIG. 1a shows the parison10 which in FIG. 1b has been partly re-formed into the parison 10' andwhich in FIG. 1c has taken on its final shape and as such constitutesthe article 11.

A mould 34 is to be found in the Figures which is particularly suitablefor the re-forming of a tube-shaped parison 10. The mould comprises anumber of component parts which may be moved relative to each other, andforming moulding elements 21, 22 and 25 which make contact with aninitial number of areas of the material 12, 13 and 20 in the parison 10.There is also a number of depressions or recesses 26, 27 in the wall ofthe mould which face a number of other areas of the material 16, 17 inthe parison 10. The element 21 is also supported by and movable relativeto the elements 22, 25 so that the elements may be moved relative toeach other in the direction of the axis of the tube-shaped parison. Theelements are held apart from each other when at rest by means ofspring-loaded elements 43, 44 so designed that the spring tension in thespring-loaded element 44 is greater that that in the spring-loadedelement 43. A further depression 35 in the wall of the mould is alsoshown in the Figures. Finally, the Figures also show a retaining element40 in which cooling ducts 42 are incorporated.

When re-forming the parison 10 into the article 11 the inside of theparison is put under pressure whereby the parison is dilated to acertain smaller extent (FIG. 1b) to ensure contact between the elements21, 22 and 25 and the initial areas of the material 12, 13 and 20 and toensure that the parison 10 is secured by the retaining element 40. Thelarge depression 35 in the wall of the mould is of such proportions thatthe excess pressure inside the parison 10 is able to dilate the parisoninto contact with the wall of the mould in the depression 35. Howeverthe pressure is insufficient to move the two other areas of the material16, 17 into contact with the wall of the mould in the depressions 26,27.

The next stage in the re-forming of the parison 10' involves thedownward movement of the element 22 as shown in FIG. 1b, whereby theelement 21 follows the movement of the element 22. Friction between theparison and the elements 21, 22 causes the areas of the material 12, 13to be drawn along with the area of the material 17 as the element ismoved downwards at the same time as the pressure inside the parisonforces the area of the material 16 into contact with the wall of themould in the depression 26, of which the axial length is reduced duringthe moulding process. An annular protuberance 30 is formed in thearticle 11 in this way (FIG. 1c). A further downward movement of theelement 22 then takes place, as shown in the Figure, when the area ofthe material 13 follows the element in its downward movement and theinternal pressure inside the parison 10' forces the area of the material17 into contact with the wall of the mould in the depression 27, ofwhich the axial length is reduced simultaneously in a similar fashion tothat which has already been described for the depression 26. An annularprotuberance 31 formed in the depression 27 in this way (FIG. 1c). Themovement of the initial areas of the material 12, 13 means that theannular protuberances 30, 31 may be moulded without stretching thematerial and without the reduction in wall thickness associated withstretching.

The principle in accordance with which a parison is re-formed into anarticle by virtue of this invention has been described above in relationto a tube-shaped parison. The idea of invention as such is, of thecourse, applicable to parisons of other shapes. In the case of a flatparison, for example, the elements 40, 25, 21, 22 are supplemented bymeans of restraints arranged on the opposite side to the flat parison.The moulding elements, for example machanical ones, move areas of thematerial into the depressions 35, 26, 27 as the parison is beingre-formed and into contact with the wall of the mould in the respectivedepression. Thus in this latter embodiment of the invention, too, themovements of the initial areas of the material 12 and 13 enableprotuberances to be formed which correspond with the annularprotuberances 30, 31. When re-forming the parison to obtain contact withthe wall of the mould in the large depression 35 re-forming only occursby stretching the material, which results in a reduction in the wallthickness. Of course the idea of the invention also includes thepossibility of taking advantage of the movement of the material even inthis latter re-forming process in order to reduce the attenuation of thematerial which would otherwise occur.

In FIGS. 2 a-c, which represent in outline the function of a device inaccordance with this invention for moulding a tube-shaped parison, anupper half of the mould 65 may be moved to an open position (FIG. 2a) bymeans of brackets 74, 75. The surfaces of the upper half of the mouldwhich make contact with those of the lower half of the mould (not shownin the Figures) are hatched for the sake of clarity. It may be seen fromthe Figures (cf. in particular FIG. 2a) that the article formed in thedevice consists of two opposing necks in preforms for use in themanufacture of bottles. Reference numerals are shown for certain of theelements which have already been described in connection with FIGS. 1a-c. The designations a and b are used in respect of the symmetricallyarranged elements so as to indicate the symmetry of the device. TheFigures also show the retaining element 40a to be attached to thecarriage component 73 with no possibility of being moved in an axialsense relative to the carriage component. Although this arrangementsimplifies the construction of the device, the invention offers thepossibility of using other combinations of stationary and movingelements in order to achieve the required relative movements between theelements.

In addition to the elements already described, FIGS. 2 a-c show asupporting framework 60 with a sliding bearing. The framework supports acarriage component 73 in which the lower half of the mould rests. Amandrel 50, which is shown in its extended position in FIG. 2a, has acentral section 51 with end sections 52, 53. The central section and theend sections are separated by spring-loaded elements 55, 56. A hydraulicunion 57 is also provided for connection to the drive unit of thecarriage component, as well as a hydraulic union 67 are connection tothe drive unit of the mandrel and a compressed fluid union 61 forsetting the internal pressure of the parison. The reference number 59designates the electrical connection for the heating device inside themandrel. The interconnected components of the mould in the upper half ofthe mould are held together by the linking elements 62, 63.

In the position shown in FIG. 2a, a tube-shaped parison is placed in thelower half of the mould and the upper half of the mould is moved to itsclosed position by a drive unit. From their positions of greatestdisengagement, which are necessary in order to permit the upper half ofthe mould to move past the end surfaces of the tube in conjunction withthe movement of the upper half of the mould to its closed position, theretaining elements 40a, b are moved towards eachother over the distance2E so as to seal the mould against the end surfaces of the tube. Themandrel is then moved by means of its drive unit into a position insidethe parison and the inside of the parison is put under pressure. At thepoint in time at which the pressure is set, the material in the parisonis at a temperature in excess of the glass-transition temperature (TG).Heating takes place either before the parison is brought to the mould orafter the parison has been placed in the mould. Alternatively, heatingof the parison before it is placed in the mould may be combined with acertain amount of post-heating inside the mould.

As pressure is applied, the parison takes on a shape which correspondswith the parison 10' in FIG. 1b. The components of the mould are thenmoved so that the shapes which correspond with the protuberance 30 inFIG. 1c are produced, followed finally by the shapes corresponding withthe protuberance 31 in FIG. 1c. The letter F is used in the Figures toindicate the movement required in order to produce the shapescorresponding with the two protuberances 30 and 31. Since two opposingpreforms are produced simultaneously, the length of the mould is reducedby the distance 2E+2F in conjunction with the moulding of the article.

FIGS. 3 & 4 show a longitudinal section through a device for mouldingtwo opposite preforms suitable for subsequent re-forming into bottles inwhich the reference characters in respect of the parison 10 and themould 34 correspond with those used previously when describing FIGS. 1 &2. FIG. 3 corresponds with FIG. 1a and FIG. 4 corresponds with FIGS. 1cand 2c.

The FIGS. 5 a-c show, in detail, the construction of the mandrel 50. Thereference characters used in respect of the mandrel 50 correspond withthe reference characters previously used in the descriptions of FIGS.2-4. In addition, the reference numerals 47 is used to indicate aheating device arranged in the central section 51 of the mandrel. Theheating device is shown in the Figure as an electric heating devicewhich is joined to the electrical connection 59 (FIG. 2b) by means ofsliding contacts 58.

The central section 51 of the mandrel is delineated by the insulatinglayer 48 which prevents heat from the central section of the mandrelfrom being transmitted to the end sections 52, 53 of the mandrel andthus to the parison in areas where no re-forming of the parison is totake place.

FIG. 6 shows the Section C--C in FIG. 3, and FIG. 7 shows the SectionA--A in FIGS. 3 and 4. FIG. 6 also incorporates the Section B--B whichcorresponds with the longitudinal section shown in FIG. 3. FIG. 6 alsoshows the division of the mould into an upper half of the mould 65 and alower half of the mould 66 already referred to above. The referencenumerals 70, 71 are used to indicate sliding bearings for the componentparts of the mould in the upper half of the mould and in the lower halfof the mould respectively. Also shown is the area of the material 16b inthe parison which has not yet been put under pressure. FIG. 7illustrates the manner in which a carriage component 73 in which themould rests is supported in the framework 60. In the carriage componentis a hydraulic cylinder 68 which drives the mandrel together withanother hydraulic cylinder 69. The latter hydraulic cylinder is thedrive unit which moves the component parts of the mould in the directionof the axis of the mould. An additional drive unit 72 is to be foundwhich links the carriage component 73 to the bracket 74 for the purposeof moving the upper half of the mould between the open and closedposition of the mould 34. A similar drive unit is connected to the otherend of the mould.

The principle of the invention has been described in relation to FIGS. 1a-c, whereas the function of a device in accordance with the inventionhas been described in relation to FIGS. 2 a-c. The detailed FIGS. 3-7represent only a clarification of FIGS. 2 a-b. Thus the description ofthe function given in relation to FIG. 2 is also applicable to thefollowing FIGS. 3-7, for which reason no new description of the functionis provided.

The concept of the invention will, of course, accommodate a good manydevices which operate in accordance with it. The device which isdescribed in detail shall therefore only be regarded as a typical devicein accordance with the invention.

What is claimed is:
 1. A method of molding an article from a tube-shapedparison comprising introducing a parison of thermoplastic materialhaving an oriented portion which is of relatively high strength andresistant to stretching into a mold having a wall with a cylindricalmold surface provided with a plurality of axially spaced recessestherein, applying internal pressure to the parison to bring a pluralityof regions of the oriented portion of the parison into contact with thecylindrical mold surface in axially spaced regions of the mold adjoiningsaid recesses such that the parison extends across the recesses withoutentering into contact with the mold wall at said recesses, andrelatively displacing said axially spaced regions of said mold in axialdirection to axially shift the plurality of axially spaced regions ofthe parison while concurrently applying the internal pressure to saidparison and reducing the axial extent of said recesses to a finalposition in which said recesses are of ring-shape with a definedcross-sectional profile, the relative axial displacement of said regionsof the mold in combination with the reduction of the axial extent of therecesses and the internal pressure applied to the parison causing saidregions of the parison in contact with the mold under the application ofinternal pressure to be correspondingly relatively axially displacedwith said regions of the mold while the regions of the oriented portionof the parison extending across said recesses are deformed into saidrecesses under said internal pressure without substantial stretching andreduction in wall thickness to assume the shape of said recesses in saidfinal position and form a ring-shaped profile on said article.
 2. Amethod as claimed in claim 1 wherein said ring-shaped profile is hollow.3. A method as claimed in claim 2 wherein said parison has at least oneopen end which is initially brought into contact with the mold wall uponapplication of internal pressure to the parison.
 4. A method as claimedin claim 2 wherein said mold is formed with a plurality of independentcylindrical elements which form said axially spaced regions.
 5. A methodas claimed in claim 2 wherein the distance of relative displacement ofsaid regions of the mold corresponds to the deformation of said regionsof the parison extending across said recesses to achieve deformation ofsaid regions of the parison while substantially maintaining thethickness thereof.
 6. A method as claimed in claim 2 wherein saidregions of the parison which are deformed into said recesses areinitially oriented substantially monoaxially in the axial direction ofthe parison.
 7. A method as claimed in claim 2 wherein said parison isheated prior to molding to a temperature in excess of the glasstransition temperature.
 8. A method as claimed in claim 2 wherein afurther portion of said parison is molded to form a part of saidarticle, the temperature of the parison during formation of saidring-shaped profile being greater than the temperature of said portionof the parison during molding.
 9. A method as claimed in claim 2 whereinsaid parison is formed with two opposing necks for two future preformswhich in turn are formable into containers.
 10. A method as claimed inclaim 2 wherein said thermoplastic material of said parison is apolyester or a polyamide.
 11. A method as claimed in claim 2 whereinsaid thermoplastic material of said parison is polyhexamethyleneadipamide, polycaprolactam, polyhexamethylene sebacamide, polyehtylene2,6- and 1,5-naphthalate, polytetramethylene, 1,2-dioxybenzoate orcopolymers of ethylene terephthalate and ethylene isophthalate.
 12. Amethod of molding an article from a tube-shaped parison of thermoplasticmaterial in which the parison is introduced into a mold having a wallwith a cylindrical mold surface provided with a plurality of axiallyspaced recesses therein, said method comprising applying internalpressure to the parison to bring a plurality of regions of the parisoninto contact with the cylindrical mold surface in axially spaced regionsof the mold adjoining said recesses such that the parison extends acrossthe recesses without entering into contact with the mold wall at saidrecesses, and relatively displacing said axially spaced regions of saidmold in axial direction to axially shift the plurality of axially spacedregions of the parison while concurrently reducing the axial extent ofsaid recesses to a final position in which said recesses are ofring-shape with a defined cross-sectional profile, the relative axialdisplacement of said regions of the mold causing said regions of theparisons in contact therewith to be correspondingly relatively axiallydisplaced to cause the portions of the parison extending across saidrecesses to be deformed into said recesses to assume the shape of saidrecesses in said final position and form a ring-shaped profile on saidarticle, said thermoplastic material of said parison being polyethyleneterephthalate, said portions of the parison which are deformed into saidrecesses having been oriented substantially monoaxially by axialstretching of the material about three times prior to formation of saidring-shaped profile.
 13. A method as claimed in claim 12 wherein saidportions of said parison which are deformed are at a temperature ofbetween 70° and 150° C.
 14. A method as claimed in claim 13 wherein saidportions of said parison which are deformed are at a temperature between90° and 130° C.